In the past, heat-sealable webs for use in such applications as infusion packaging, desiccant bags, medical packaging and the like have been made utilizing wet-forming paper-making technology. These webs are typically reinforced by adding long natural or synthetic fibers to a pulp fiber furnish so the webs have adequate wet strength. Because such long fibers are difficult to handle in wet-forming systems, viscosity modifiers (e.g., guar gums and the like) are often added to improve uniformity of the resulting web. These additives also provide improved levels of wet strength.
Heat-sealability is typically provided by a second wet-formed layer containing a relatively large proportion of thermoplastic heat-seal fibers such as, for example, vinyl acetate, polyethylene or polypropylene fibers. Such conventional wet-formed webs are relatively expensive at least because of the high levels of both long fibers and heat-seal fibers as well as relative low production rates because of the difficulty in handling long fibers in a wet-forming system.
Accordingly, there is still a need for an inexpensive, high strength, nonwoven fabric which is able to be heat-sealed. There is also a need for an inexpensive heat-sealable composite fabric which is able to resist delaminating even after exposure to water, aqueous solvents, oils and the like. A need exists for an inexpensive heat-sealable composite fabric that can be used as a material for infusion packages or as a permeable component of infusion packaging. There is also a need for a practical method of making an inexpensive heat-sealable composite fabric. This need also extends to a method of making such a composite fabric which contains pulp fibers and continuous spunbonded filaments of a thermoplastic polymer. Meeting this need is important since it is both economically and environmentally desirable to substitute ordinary pulp fiber for high-quality exotic pulps and expensive heat-seal fibers and still provide an inexpensive heat-sealable composite fabric.